This invention relates to coating electrode surfaces, and in particular to a coating method which minimizes filament formation in surge limiters.
Surge limiters have for many years been used to protect apparatus from high voltage surges resulting from various causes, such as lightning strikes. The devices basically comprise a pair of electrodes with a spark gap therebetween. The device, which is coupled in parallel with the protected apparatus, is nonconducting during normal operation of the apparatus. However, when a voltage surge of sufficient magnitude appears at the electrodes, a spark is produced across the gap and the surge is shunted from the protected apparatus. In the sealed gas surge limiter, the electrodes are placed in a hermetically sealed housing along with an inert gas. The device fires when the gas in the gap area is sufficiently ionized to produce a spark.
It has been recognized in such devices that a coating of graphite on the surface of the electrodes will improve device performance by increasing electron emission from the electrode and thereby enhancing formation of the plasma discharge in the gap. One problem associated with such coatings, however, is the formation of carbon filaments on the surface after a few discharges of the device, which results in leakage currents and could produce short circuits in extreme cases.
It is, therefore, a primary object of the invention to provide a coating on the surface of the electrodes with sufficient bonding therebetween so that filament formation is minimized.